1. Field of the Invention
This invention relates to spherical phosphor aggregates, their preparation and their use in X-ray screens. More particularly, this invention relates to spherical aggregates of needle or plate shaped phosphor particles produced by spray drying a slurry of said particles and a binder and the use of such aggregates in radiographic imaging systems.
2. Background of the Disclosure
The use of phosphors for radiographic imaging systems is old and well known to those skilled in the art. One type of X-ray imaging system presently used employs an X-ray screen construction of the type disclosed in U.S. Pat. No. 4,316,092 wherein a photographic film employing a light sensitive emulsion on both sides of the film is placed between two X-ray screens, each of which contains a suitable phosphor. A patient or other object to be X-rayed is placed between the film-containing X-ray screen cassette and a source of X-rays. The X-rays penetrate the object being X-rayed and strike the phosphor particles in the screen. This causes the phosphor to emit radiation in the visible and/or UV spectrum which strikes the photographic film which, when developed, yields the desired image.
Other types of X-ray imaging systems include various latent image recallable radiographic imaging systems employing a photoluminescent or thermoluminescent phosphor medium to store a latent image produced therein from exposure to high energy radiation, such as X-rays and the like. In medical radiographic applications, X-rays from a suitable X-ray source are passed through the patient and impinge upon the phosphor medium to produce an immediate first light image which may, if desired, be recorded on photographic film. A recallable, latent image is also produced in the phosphor and remains in the phosphor medium until recalled by the application of a suitable energy source such as a laser. This stimulates the phosphor thereby producing a second light image by thermoluminescent response or photoluminescent response.
U.S. Pat. No. 3,996,472 discloses a phosphor useful in radiation dosimeters which exhibits thermoluminescent response when subjected to heat stimulation. U.S. Pat. Nos. 4,346,295 and 4,356,398 disclose light emitting laser means as the source of stimulating energy to produce a photoluminescent response in various phosphor materials. U.S. Pat. No. 4,481,416 discloses a radiographic imaging system which utilizes a thermoluminescent phosphor layer in combination with a laser stimulated read-out system to generate the energy for exciting the phosphor in order to produce the final desired light image from the latent image stored in the phosphor. A moving laser beam scans the phosphor medium to activate the phosphor to produce the visible light which is then read by a suitable scanning device.
The ideal phosphor particle should be in the form of a perfect sphere in order to minimize cross-talk and light piping parallel to the film. Unfortunately some useful phosphors, such as lanthanum oxybromides, are in the form of needle or plate shaped crystalline particles wherein one dimension is significantly larger than one or more of the other dimensions. In the preparation of X-ray and other radiographic imaging screens, a phosphor is slurried in a liquid medium having a suitable binder material dissolved in the liquid and the slurry is applied as a coating to a suitable base or substrate, such as a film or sheet of polyester (i.e., Mylar), by use of a draw-down or other doctor blade technique in order to achieve a uniform coating of the phosphor on the substrate. This causes the plate and needle shaped phosphor particles to line up in a direction such that the largest dimension is parallel to the longitudinal direction of the substrate. This type of alignment with such phosphor particles results in the worst light piping and scattering problems that one can achieve with them, and also creates substantial reflection problems.
One such attempt to overcome this alignment problem is disclosed in U.S. Pat. No. 4,498,008 where various irregularly shaped doctor blades are employed, along with a sinusuidal movement of the blade, to reduce particle alignment in at least one direction. Other attempts have been directed towards trying to form such inherently needle or plate shaped phosphor particles more in the form of polyhedral or square shaped particles, as is disclosed in U.S. Pat. No. 4,315,979.
U.S. Pat. No. 4,315,979 relates to rare earth oxyhalide phosphors of improved brightness, wherein the plate-shaped phosphor particles have a length to thickness ratio (also known as the aspect ratio) no greater than about 10 to 1. These phosphors exhibit less light scattering and absorption by reason of the more polyhedral crystalline characteristics, in comparison with the same phosphors having aspect ratios which often exceed 15 to 1. U.S. Pat. No. 4,316,092 discloses an X-ray screen containing a mixture of two different rare earth phosphors useful for green or blue-sensitive radiographic films. One of the phosphors is a plate-shaped, thulium activated lanthanum oxyhalide and the other is a polyhedral-shaped, lanthanum and gadolinium oxysulfide activated with terbium. The plate-like phosphor particles in the mixture are more randomly oriented in the phosphor layers due to the presence of the generally larger size and polyhedral-shaped oxysulfide phosphor particles.
The most ideal situation for X-ray or other radiographic screens employing needle or plate shaped phosphor particles would be one wherein the needle or plate shaped particles are aligned in a manner such that the largest dimension of each phosphor particle is perpendicular to the longitudinal direction of the substrate, such as the teeth of a comb. It has not yet been possible to achieve this result. Accordingly a need still exists for improved radiographic screens employing needle or plate shaped phosphor particles.